436 Birge—Vertical Distribution of Pelagic Crustacea. 
us to figure our numbers in terms of individuals per square 
meter of area. Secondly, the percentages seemed to us to offer 
an easier and fairer basis of comparison for our purposes than 
do the actual numbers. In the case of the more abundant Crus¬ 
tacea, the numbers per square meter rise into the millions, and 
are therefore inconvenient for basis of comparison. Again: In 
lakes of different character the total number of Crustacea will 
vary greatly, so that comparison between lake Mendota and 
other bodies of water would not be easy. A comparison of the 
percentile distribution of the Crustacea is, however, easily made, 
and is or should be measurably independent of the actual num¬ 
ber of Crustacea per square meter. 
In determining the average departure of each observation from 
the average of the period, the difference between the number of 
Crustacea found in each observation and the average for the 
period was determined, and the sum of these departures was 
divided by the total number of observations. In determining 
the average percentile departure in each level a similar method 
was followed. In these determinations, however, only those ob¬ 
servations could be used in which a complete series was present. 
The average per cent, used for this purpose is therefore some¬ 
what different from the percentage which is given as the aver¬ 
age of the period, since in that were included partial series of 
observations as well as complete ones. 
METEOROLOGICAL CONDITIONS. 
In planning our work it was thought best to distribute our 
observations over several periods of two or three days each, ex¬ 
tending through a month, in order that the distribution of the 
Crustacea by day and night might be tested under different con¬ 
ditions of weather, and under different aspects of the moon. The 
observations have been divided into four sets, known as periods. 
Of these the first, Period I, includes the week from July 7 to 
July 14, when observations were being made to test the work¬ 
ing of the dredge and our methods for counting. A number of 
observations made at this time were failures for various rea¬ 
sons, and were rejected. Nine series, which havejoeen retained, 
were successful. They were taken at various times of the day, 
